The invention pertains to a mold for the production of a component of quartz glass or high-silica glass. The mold has a bottom and a mold wall, consisting of a material with a coefficient of thermal expansion greater than that of quartz glass. This wall forms the boundary of a cavity provided with a fill opening and is divided vertically into several segments, which are connected to each other in such a way that they can separate under the action of a force acting outward from the cavity. The mold also has a pretensioned elastic element installed outside the cavity, this element being in effective contact with at least one segment so that, as a result of the pretension, an elastic force acts on the segment in the direction toward the cavity.
A mold of this type is known from FR-PS 1,352,564. Here a cylindrical graphite mold is described, which has a bottom, on which a sleeve-like mold wall is placed. The mold wall is divided lengthwise into several segments, which are connected detachably to each other. The longitudinal edges of the individual segments are designed to form lap joints, so that the individual segments, when placed next to each other, form a mold wall, which, although closed, still has a certain resilience with respect to forces acting from the interior of the mold toward the outside. The bottom of the mold is designed with a vertically upward-projecting collar, which extends outside and around the mold wall. On the opposite end of the sleeve-like mold, the individual segments are held together by a retaining ring, which is also provided with a vertically oriented collar, extending outside and around the mold wall. The individual segments of the mold wall are also held together by corrugated spring rings made of molybdenum wire bent into serpentine shape, which are stretched around the outside circumference of the mold wall. The coefficient of thermal expansion of quartz glass is much smaller than that of graphite. When cooled from high temperatures, the graphite mold therefore shrinks onto the quartz component which has been produced. It is possible for the graphite mold or the quartz glass component to break during this shrinking phase. The purpose of dividing the mold wall into segments is to take advantage of the resilience of the lap joints in the outward direction so that the individual segments are able to give way when the graphite mold shrinks onto the glass and so that the mold is therefore unable to trans- mit any forces to the quartz glass component.
To produce a quartz glass component, the mold is usually filled with quartz crystal grains and then heated. When molybdenum is heated to temperatures above 1,100.degree. C., however, it reacts with carbon and becomes, in effect, case-hardened. As this process continues, the corrugated molybdenum rings lose their elasticity. Their pretension decreases, and they start to slide down along the outside wall of the mold. The individual segments of the mold wall are then held together only by the ring-shaped collars at the two ends of the mold. The time at which the corrugated rings loose their pretension to this extent cannot be predicted precisely. Upon further heating, the viscosity of the quartz glass decreases progressively, so that glass melt now intrudes between the resilient lap joints between the individual segments and can even emerge to the outside. The cooling of the mold and of the quartz glass melt proceeds inward from the outside wall. The quartz glass which has intruded into the lap joints solidifies first. Because of the quartz glass which has intruded into the joints and solidified there, however, it is no longer certain that the lap joints between the individual segments will be sufficiently resilient. The graphite mold can therefore shrink onto the quartz glass component, and the forces which thus arise can lead to the breakage of the mold or of the glass.